The invention relates to a tangential on board injector with an auxiliary supply of further cooled compressed air, from an external heat exchanger or air cooled bearing gallery for example, that serves to reduce the flow quantity requirements for cooling air to cool a rotor and blades in a gas turbine engine.
The invention is applicable to gas turbine engine cooling systems and in particular an improved supply arrangement for cooling air flow to regulate the operating temperature of the turbine blades.
It is widely recognised that the efficiency and energy output of a gas turbine engine can be increased by increasing the operating temperature of the turbine. Under elevated operating temperatures, gas turbine engine components such as the turbine rotors and blades are cooled by a flow of compressed air discharged at a relatively cool temperature. The flow of coolant across the turbine rotor and through the interior of the blades removes heat so as to prevent excessive reduction of the mechanical strength properties of the blades and rotor.
Therefore on the one hand the turbine operating temperature, efficiency and output of the engine are limited by the high temperature capabilities of the various turbine elements and the materials of which they are made. In general the lower the temperature of the elements the higher strength and resistance to operating stresses. On the other hand the performance of the gas turbine engine is very sensitive to the amount of air flow that is used for cooling the hot turbine components. The less air that is used for cooling functions the better the efficiency and performance of the engine.
To cool the turbine rotor blades, a flow of cooling air is typically introduced at a low radius as close as possible to the engine centreline axis. The cooling flow is introduced with a swirl or tangential velocity component through use of a tangential on board injector (TOBI) with nozzles directed at the rotating hub of the turbine rotor.
The effectiveness of cooling air flow is enhanced if the temperature of the cooling air flow is reduced in comparison to the gas path temperature. Cooling air flow is generally derived directly from the output of the compressor without additional processing. The temperature of air increases as it is compressed, however, the compressed air remains below the temperature of the air within the combustor and turbine gas path resulting in the capacity to cool the turbine rotor and turbine blades.
Of course, the lower the temperature of the cooling air flow, the less flow amount that is required for the same cooling effect. Most prior art gas turbine engine systems, however, accept the practical limitation of cooling air at the temperature at which it is delivered from the compressor. Designers merely increase the flow amount of the compressed air to increase the cooling of the turbine components.
The tangential on board injector intakes compressed air from the compressor and delivers the air directed towards rotating rotor hub components with a swirl or tangential velocity component. As the cooling air flow passes over a rotating turbine hub, the air flow temperature rises due to the pumping of the flow from the low injection radius near the engine centreline to the high radius at the turbine blade entry area. In effect the rotating turbine hub acts as an impeller and pumps the air from the injection radius close to the engine centreline. As air is forced radially outward by the rotation of the turbine, the temperature rises as a result of the compression of air during radial pumping as well as the absorption of heat from proximity to the rotor.
By introducing air flow from the tangential on board injector at a swirl or tangential velocity, the temperature rise in the cooling air flow caused by the pumping phenomenon is reduced. In order to introduce a swirl in the air flow from the tangential on board injector, a radial injector conventionally includes an array of injector blades spanning between a forward injector wall and a rearward injector wall to define main-flow nozzles disposed in a circumferential array for directing a main compressed air flow tangentially radially inwardly. Therefore in the tangential on board injector, compressed air is re-directed by the injector blades through TOBI nozzles to direct air with a swirl or tangential velocity component towards rotating turbine rotor components which are to be cooled. The tangential velocity of the injected air flow is generally greater than the rotational velocity of the turbine rotor in order to enable efficient movement of the cooling air flow relative to the rotating rotor.
In the prior art the temperature of the compressed air available for cooling functions is not variable or under the direct control of the designer. Compressed air is delivered from the compressor at a given temperature that is lower than the gas path temperature and therefore may be used for cooling. In order to control the cooling capacity of this compressed air flow, designers increase or decrease the volume of air flow but in the prior art have not to date adjusted the temperature.
In the prior art however, in order to remove heat from the bearings supporting the rotating shafts of an engine, an external heat exchanger is used to deliver cooling air to the bearing gallery. The relatively small amounts of cooling air delivered to the bearings by an external heat exchanger can be carefully controlled and introduced to the bearing gallery at a wide range of selected temperatures. To-date however, the prior art does not include any external heat exchanger input to the air flow conducted over rotor turbines and blades.
It is an object of the invention to provide greater control over the amount and temperature of cooling air flow delivered by a tangential on board injector to turbine rotors and blades.
It is a further object of the invention to adapt the heat exchanger derived cooling air directed to bearing galleries to improve the control and delivery of cooling air from a tangential on board injector system.
It is a further object of the invention to provide a tangential on board injector with reduced drag due to ejection of auxiliary air by the injector blades.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
A tangential on board injector with auxiliary supply of further cooled compressed air, from an external heat exchanger or air cooled bearing gallery for example, serves to reduce the volume of cooling air directed tangentially toward a cooled rotor of a gas turbine engine.
The tangential on board injector has an array of injector blades between two injector walls defining circumferential main flow nozzles for directing a main compressed air flow tangentially radially inwardly. The invention is equally applicable to radial and axial TOBI configurations since each includes injector blades.
Each blade has an interior chamber in flow communication with a source of auxiliary compressed air with at least one bore extending between the chamber and an exterior surface of the blade. The bores eject further cooled air from the heat exchanger and merge with the primary compressed air flowing through the injector nozzles. The bores may also produce a cooling film of air that reduces drag of the injector blades.
The introduction of relatively cooler compressed air ejected through the hollow TOBI blades and cooling bores results in several advantages. The auxiliary air supply from an external heat exchanger adds only marginal cost to the engine since many conventional engines include cooling air supply to the bearing gallery adjacent the TOBI. By merely extending the cooling air supply conduit from the bearing gallery to the TOBI blade area, and increasing the volume of air flow marginally, further cooling air can be supplied to the TOBI at very little cost.
The advantages include a controllable reduction in the tangential on board injector cooling air temperature and a corresponding reduction in the amount of cooling air flow required. The auxiliary supply of cooled air from a heat exchanger adds a significant degree of control over injector flow amount and temperature that enables fine tuning of the delivery of cooling air to the rotor blades. For example, the heat exchanger can be configured to deliver additional cooling air at a predetermined temperature and flow amount. As a consequence of the improved control over delivery of cooling air the durability and service life of air cooled rotor blades is enhanced. When multiple bores are used to deliver additional cooling air, an air film is created by the ejected air over the injector blades especially in the area of trailing edges resulting in reduced drag losses through the TOBI and reduced demand on the compressor.
Further advantages of the invention will be apparent from the following detailed description and accompanying drawings.